Viscoelastic fluids play a very important roles in oilfield applications. The viscosity allows the fluids to carry particles from one place to another. For example, the drilling fluid is able to carry the drilling cuts from the wellbore to the surface. Viscous fluids also play an essential role in gravel packing completion. In gravel pack operations, a steel screen is placed in the wellbore and the viscous completion fluid places prepared gravel of a specific size in the surrounding annulus to minimize the sand production. Fracturing fluids are also required to be viscous enough. A hydraulic fracture is formed by pumping the fracturing fluid into the wellbore at a rate sufficient to increase pressure downhole to exceed that of the fracture gradient of the rock. The fracturing fluid contains the proppant, which keeps an induced hydraulic fracture open after the pressure is released. Therefore it is important for the fluid to have enough viscosity to transport the proppant into the fracture.
Polymers have been used to make viscous fluids for decades. However, recently, viscoelastic surfactants (VES) have been widely applied to the oilfield in applications such as drilling, gravel packing, acidizing, and fracturing applications due to their non- or less-damaging characteristics. VES-based fluids have excellent capacity to suspend and transport sand/proppant. VES fluids have several distinctive advantages over polymer-based fluids. Unlike polymer fluids, the VES based fluids are solid free, which minimize the formation damage after they break. However, many viscoelastic surfactants are very sensitive to high concentrated brines. They don't often gel the heavy brines or the fluid viscosity is not stable under high temperature conditions. Therefore, viscoelastic fluids have some limitations for drilling, completion and fracturing applications, especially for deep wells, because many deep wells have bottom hole temperatures of 149° C. (300° F.) or more, and they require heavy fluids to balance the well pressure and maintain control of the well.
In the literature, it has been reported that several VES packages, such as VES/low MW polymer, cationic/anioinic surfactants and VES/cosurfactant can successfully viscosify moderate density brines (like CaCl2, CaBr2 and NaBr brine). However, none of them can work in heavy ZnBr2 brine at temperatures above 250° F. under normal dosage (equal or less than 6 vol % as received). The ZnBr2 brine and the mixed brine made by ZnBr2/CaBr2/CaCl2 will be used if a density of 15 ppg or higher is needed for deep wells to balance the well pressure.
U.S. Patent Application Publication No. 2002-0033260 describes a high brine carrier fluid having a density of >1.3 g/cm3 (10.8 ppg) contains a component selected from organic acids, organic acid salts, and inorganic salts; a cosurfactant that may be sodium dodecylbenzene sulfonate (SDBS), sodium dodecylsulfate (SDS) or a mixture of two, or a hydroxyethylaminocarboxylic acid; and a zwitterionic surfactant, preferably a betaine, most preferably an oleyl betaine. It is indicated that zinc halides are not preferred, especially zinc bromide. In the examples, the heaviest brine that a useful viscosity was maintained in was at a density of 1.64 g/cm3 (13.7 ppg). The highest working temperature is 138° C. (280° F.).
U.S. Pat. No. 7,148,185 B2 describes the surfactant fluid gels that are stable to brines having densities above about 1.56 g/cm3 (13 ppg) at high temperatures. The well treatments fluids contain a surfactant, preferably erucylamidopropyl betaine, and an amount of alcohol, preferably methanol, and a salt or mixture of salts of a divalent cation or mixture of divalent cations forming a brine, preferably one or more of bromide and/or chlorides of calcium and/or zinc. Cosurfactants, such as sodium dedecylbeneze sulfonate (SDBS) can also be used. The concentration of surfactant, BET-E-40, shown in the most of examples in heavy brines are 10%.
The VES fluid/fluid system of the present invention addresses the problem that drilling and production engineers have had for years. More particularly, the VES based fluid system of the invention exhibits significantly improved viscosity in high-density brines at elevated temperatures (>300° F.).